A large number of liquid crystal devices have been used as a screen display device for a computer device, and are expected to also be used more in the future for TVs. However, currently widely-available liquid crystal display devices are those in TN (Twisted Nematic) mode, having a narrow viewing angle and an insufficient response rate. This causes problems such as reduction in contrast due to parallax and blurs in moving pictures, posing a large problem about the display capability in TN mode for TVs.
In recent years, studies have been directed to the OCB mode instead of the TN mode. The OCB mode is a scheme with a wide viewing angle and a high-speed response compared with the TN mode. Therefore, the OCB mode is a display mode more suitable for nature motion picture display.
Hereinafter described are a liquid crystal display device and a method of driving the same that adopt the OCB mode.
FIG. 1 illustrates the construction of a conventional liquid crystal display device. In FIG. 1, the liquid crystal display device includes gate lines GL1 through GLn, source lines SL1 through SLm, a plurality of thin-film transistors (hereinafter referred to as TFTs) 103 as switching devices, etc. FIG. 2 illustrates a picture element section thereof. As illustrated in FIG. 2, a drain electrode of the TFT 103 is connected to a picture-element electrode in a picture element 104. The picture element 104 is structured by the picture-element electrode, a common electrode 201, a liquid crystal 202 held between both of these electrodes, and a storage capacitor 203 formed between the picture-element electrode and the common electrode 201. The common electrode 201 is driven by a voltage supplied by a common driving section 105 illustrated in FIG. 1.
In FIG. 1, the gate driver 101 applies a voltage to the gate lines GL1 through GLn for turning the TFTs 103 ON or OFF. In synchronization with data supply to the source lines SL1 through SLm, the gate driver 101 sequentially applies an ON potential to the gate lines GL1 through GLn.
The source driver 102 applies a voltage to the source lines SL1 through SLm to supply the voltage to the respective picture elements 104. A difference between the voltage supplied to the common electrode 201 and the voltage supplied to each of the source lines SL1 through SLm to be applied to the picture element 104 is a voltage between both ends of the liquid crystal 202 in the picture element 104. This voltage determines the transmittance of the picture element 104.
The above driving scheme is applied not only to OCB cells, but also to a case where TN-type cells are used. When the OCB cells are used, however, an activation step of starting video display requires unique driving, which is not required when the TN-type cells are used.
An OCB cell has a bend configuration enabling image display or a spray configuration disabling imaged display. To cause transition from the spray configuration to the bend configuration (hereinafter referred to as transition), the unique driving is required, such as applying a high voltage for a predetermined time. Note that the driving associated with transition is not directly related to the present invention, and therefore is not further described herein.
This OCB cell has a problem that, even once transition is made to the bend configuration by the above unique driving, if a voltage over a predetermined level has not been applied for over a predetermined time, the bend configuration cannot be maintained and is back to the spray configuration. This phenomenon is hereinafter called “back transition”.
To suppress the occurrence of back transition, as disclosed in Japanese Patent Laid-Open Publication No. 11-109921 and Japanese Liquid Crystal Society Journal, Apr. 25, 1999 (Vol. 3, No. 2) P.99 (17) through P.106 (24), it is known that a high voltage is regularly applied to the OCB cells. Hereinafter, driving for periodically applying a high potential in order to suppress back transition is called “anti-back transition driving”.
FIG. 3 illustrates potential-transmittance curves observed in general OCB.
A curve 301 is a potential-transmittance curve at normal driving, not at anti-back transition driving, and a curve 302 is a potential-transmittance curve at anti-back transition driving. A potential 303 indicates a critical potential Vth at which back transition occurs at normal driving. A potential 304 is a potential when the transmittance is at the highest (white potential), and a potential 305 is a potential when the transmittance is at the lowest (black potential).
At normal driving (that is, when no prevention of back transmission is carried out), the configuration of the OCB cell is back to the spray configuration when the potential becomes Vth or lower, and therefore an appropriate transmittance cannot be obtained. Thus, driving is always made with a potential not lower than Vth. In such case, however, as illustrated in FIG. 3, sufficient luminance cannot be obtained. For this reason, the OCB requires anti-back transition driving to be carried out.
As is well known, liquid crystals typified by OCB and TN require so-called alternating driving to be carried out. However, the above-described Japanese Patent Laid-Open Publication No. 11-109921 and Japanese Liquid Crystal Society Journal do not disclose any specific construction of a liquid crystal display device in OCB mode. Therefore, both of the documents do not help specify which type of alternate inversion should be carried out. Therefore, hereinafter described is a virtual example of anti-back transition driving when the most general alternating driving (that is, a combination of line-by-line inversion and frame-by-frame version) is carried out.
FIG. 4 is an illustration showing the construction of a liquid crystal display device as the virtual example. In FIG. 4, the liquid crystal display device includes a signal converting section 401, a driving pulse generating section 402, a source driver 403, a gate driver 404, and a liquid crystal panel 405. The signal converting section 401 doubles the speed of an input image signal line by line, converting it into a double-speed signal composed of a double-speed image signal and a double-speed non-image signal. The driving pulse generating section 402 generates pulses for driving the respective drivers 403 and 404. To facilitate understanding of the description, assume for convenience sake that the number of source lines of the liquid crystal panel 405 is ten (SL1 through SL10), the number of gate lines is ten (GL1 through GL10), and one frame period is composed of ten horizontal periods.
Described next is an operation of anti-back transition driving to be carried out by this liquid crystal display device. An input image signal is doubled in speed line by line in the signal converting section 401, and is then supplied to the source driver 403.
FIG. 5 illustrates a specific construction of the signal converting section 401. Also, FIG. 6 illustrates timing of a converting operation. The control signal generating section 501 generates various control signals, such as a clock, from a synchronizing signal synchronized with the input image signal. The input image signal is written in a line memory 502 in synchronization with a write clock from a control signal generating section 501. The image signal written in the line memory 502 is read from the line memory 502 in synchronization with a read clock (having a frequency twice as much as that of the write clock) from the control signal generating section 501, and this reading is carried out during a period half of that of writing. While the image signal is being read from the line memory 502, the output signal selecting section 504 selects this image signal as an output and, during the remaining period, selects a non-image signal outputted from the non-image signal generating section 503 as an output. Consequently, as illustrated in FIG. 6, in one horizontal period of the input signal, the double-speed non-image signal and image signal are outputted in time sequence. The non-image signal is a signal for applying a predetermined high potential to the OCB cells with predetermined periodicity.
In FIG. 4, the source driver 403 alternately inverts the output signal (double-speed signal) from the signal converting section 401 for supply to the source lines (SL1 through SL10) of the liquid crystal panel 405. FIG. 7 is an illustration showing timing of a polarity control signal and driver driving pulses when line-by-line inversion and frame-by-frame inversion are combined as described above. The polarity control signal, which is to switch alternating polarity, is a signal obtained by XORing a frame inverting signal (A) and a line inverting signal (B), and is generated by the driving pulse generating section 402 illustrated in FIG. 4.
An input-output characteristic of the source driver 403 is illustrated in FIG. 8. In FIG. 8, signal outputs higher with respect to a reference potential are illustrated as having a positive polarity, and those lower with respect thereto as having a negative polarity. Also, in FIG. 7, this polarity is represented as “+” or “−” in each gate-selected period. For example, “+” is indicated on a gate pulse P1 at a location corresponding to a period T0_1 during which the gate pulse P1 is selected. This indicates that a voltage supplied by the source driver 403 during the period T0_1 has a positive polarity. As illustrated in FIG. 8, the source driver 403 supplies a positive voltage when the polarity control signal is HI, and supplies a negative voltage when LOW.
In FIG. 7, gate pulses P1 through P10 are pulses for selecting ten gate lines (GL1 through GL10), respectively, on the liquid crystal panel 405 during their HI periods. The gate pulses P1 through P10 are driven in the following manner in accordance with timing of the double-speed signal inputted to the source driver 403.
During the period T0_1 illustrated in FIG. 7, the gate pulse P1 becomes HI, and a positive image signal S1 is written in picture elements on the gate line GL1. During the following period T0_2, the gate pulse P7 becomes HI, and a negative non-image signal is written in picture elements on the gate line GL7. During a period T0_3, the gate pulse P2 becomes HI, and a negative image signal S2 is written in picture elements on the gate line GL2. During the following period T0_4, the gate pulse P8 becomes HI, and a positive non-image signal is written in picture elements on the gate line GL8. Thereafter, signals are sequentially written in accordance with the polarity of the polarity control signal.
In this way, each of the gate lines (GL1 through GL10) on the liquid crystal panel 405 is selected twice during one frame period. In the picture elements on each gate line, an image signal and a non-image signal are written once.
During a period T1_1 of a second frame coming next, the gate pulse P1 becomes HI, and a negative image signal S′1, which is opposite in polarity of the signal in the first frame, is written in the picture elements on the gate line GL1. During the following period T1_2, the gate pulse P7 becomes HI, and a positive non-image signal, which is opposite in polarity of the signal in the first frame, is written on the gate line GL7. Thereafter, similarly, signals opposite in polarity to those in the first frame are sequentially written.
As such, the image signal is made opposite in polarity to that in the previous frame so as to avoid sticking caused on the liquid crystal panel when signals of the same polarity are retained for a long time.
With the above-described operation , it is possible to write image signals as well as to periodically write non-image signals. By giving voltages of the non-image signals as appropriate, back transition can be prevented.
In FIG. 7, however, when changes in polarity of the signals (image signal and non-image signal) written in each line are noted, as to the first line through the fifth line, after an image signal is written, a non-image signal (B) opposite in polarity to this image signal is always written. Furthermore, after the non-image signal is written, an image signal equal in polarity to the non-image signal is always written. On the other hand, as to the sixth line through the tenth line, after an image signal is written, a non-image signal equal in polarity to the image signal is always written. Furthermore, after the non-image signal is written, an image signal opposite in polarity to the non-image signal is always written. That is, the phase relation is changed between the first line through fifth line and the sixth line through the tenth line.
Such change in the polarity inversion relation at a certain line will affect charging to the liquid crystal and, consequently, become a cause of impairing evenness in image quality. Especially in recent years, liquid crystal panels have become upsized and capable of displaying with higher definition. Accordingly, wiring resistance in a glass substrate becomes increased, and also a time allocated for recharging each picture element tends to be shorter. Therefore, influences caused by the change in the phase relation on recharging the picture elements are not negligible, despite technologies for improving the capability of a picture element transistor, etc. Therefore, in the above virtual example, a difference in luminance is disadvantageously recognized between the fifth line and the sixth line.
Furthermore, compared with normal driving where non-image signals are not inserted, the driving frequency in the above virtual example becomes double. Therefore, the time allocated for writing the image signal to each picture element is shortened by half compared with normal driving. Consequently, writing of data to the picture elements may not sufficiently be carried out.
Therefore, an object of the present invention is to provide a liquid crystal display device and a method of driving the same that can suppress the occurrence of back transition without causing the above problems when OCB cells are used and, as a result, can display a good image.